Rail and method for producing same

ABSTRACT

A rail exhibits a high 0.2% proof stress after shipping, which is effective for improving rolling contact fatigue resistance of the rail, the rail having a chemical composition containing C: 0.70% to 0.85%, Si: 0.1% to 1.5%, Mn: 0.4% to 1.5%, P: 0.035% or less, S: 0.010% or less, and Cr: 0.05% to 1.50%, with the balance being Fe and inevitable impurities, and exhibiting, at least 90 days after a preparation date of a steel material inspection certificate of the rail which describes at least a measurement result of a 0.2% proof stress of a head of the rail, an improvement margin of a 0.2% proof stress of 40 MPa or more, relative to the 0.2% proof stress described in the steel material inspection certificate.

TECHNICAL FIELD

The disclosure relates to a rail, particularly a high-strength pearliticrail, and to a method for producing the same. Specifically, because thiskind of rail is used under severe high axle load conditions such as inmining railways which are weighted with heavy freight cars and oftenhave steep curves, the disclosure provides a method for providing ahigh-strength pearlitic rail having excellent rolling contact fatigueresistance which is suitable for prolonging the rail service life.

BACKGROUND

In heavy haul railways mainly built to transport ore, the load appliedto the axle of a freight car is much higher than that in passenger cars,and rails and wheels are used in increasingly harsh environments. Forsuch a rail used in heavy haul railways, specifically, in railways onwhich trains and freight cars run with high loading weight, steel havinga pearlite structure is conventionally primarily used, from theviewpoint of the importance of rolling contact fatigue resistance. Inrecent years, however, to increase loading weight on freight cars andimprove the efficiency of transportation, there has been demand forfurther improvement of rolling contact fatigue resistance of rails.

Consequently, there have been made various studies for furtherimprovement of rolling contact fatigue resistance. For example, JP5292875 B (PTL 1) proposes a rail having excellent wear resistance,rolling contact fatigue resistance, and delayed fracture resistance, therail having defined ratios of the Mn content and the Cr content, and ofthe V content and the N content. JP 5493950 B (PTL 2) proposes a methodfor producing a pearlitic rail having excellent wear resistance andductility, in which the pearlitic rail has defined contents of C and Cuand is subjected to post heat treatment at heating temperature of 450°C. to 550° C. for 0.5 h to 24 h. JP 2000-219939 A (PTL 3) proposes apearlitic rail having excellent wear resistance and surface damageresistance, the pearlitic rail having a defined C content and structureand further having a 0.2% proof stress of 600 MPa to 1200 MPa. JP5453624 B (PTL 4) proposes a pearlite steel rail having a 0.2% proofstress of more than 500 MPa and less than 800 MPa, the pearlite steelrail having defined contents of C, Si, Mn, P, S, and Cr, and a definedsum of contents of C, Si, Mn, and Cr.

CITATION LIST Patent Literature

PTL 1: JP 5292875 B

PTL 2: JP 5493950 B

PTL 3: JP 2000-219939 A

PTL 4: JP 5453624 B

SUMMARY Technical Problem

A rail obtained through hot rolling and accelerated cooling is typicallysubjected to straightening treatment to eliminate a bend of the rail. Inthis straightening treatment, the 0.2% proof stress is significantlydecreased by the Bauschinger effect. Specifically, to impartstraightness to a rail, for example, the rail has to be straightenedwith a load of 30 tf to 70 tf. When straightening treatment is performedwith such a high load, the 0.2% proof stress after the straighteningtreatment is significantly decreased as compared with before thetreatment.

Then, alloying elements need to be added to sufficiently enhance the0.2% proof stress before straightening treatment of a rail, but adding alarge amount of alloying elements rather causes an abnormal structureother than a pearlite structure. Thus, adding more alloying elementsthan the present level is difficult. Therefore, a decrease in the 0.2%proof stress caused by the Bauschinger effect needs to be prevented by amethod other than the addition of alloying elements.

All the techniques described in PTL 1 to PTL 4, however, merely improvethe 0.2% proof stress in a stage before a rail is subjected tostraightening treatment. Any of the techniques cannot avoid a decreasein the 0.2% proof stress after straightening treatment.

Specifically, the technique described in PTL 1 defines a ratio of the Mncontent and the Cr content, and a ratio of the V content and the Ncontent, but the rail loses the 0.2% proof stress in straighteningtreatment as described above. Thus, the 0.2% proof stress cannot besufficiently maintained after straightening treatment only by definingthe ratio of alloying elements.

PTL 2 proposes to define contents of C and Cu and to perform post heattreatment at heating temperature of 450° C. to 550° C. for 0.5 h to 24h, but the heating temperature is high only to decrease the 0.2% proofstress because of recovery of dislocation. Thus, the 0.2% proof stressis more decreased after straightening treatment.

The technique described in PTL 3 sets the C content to more than 0.85%and increases the amount of cementite, thus ensuring a high 0.2% proofstress. On the other hand, a decrease in elongation tends to causecracking, thus making it difficult to ensure rolling contact fatigueresistance.

The pearlite steel rail of PTL 4 has a 0.2% proof stress as low as lessthan 800 MPa, and actually has difficulties to ensure rolling contactfatigue resi stance.

The disclosure has been developed in light of the above circumstances.It could be helpful to provide a method for achieving a high 0.2% proofstress in a rail after shipping, which is effective at improving rollingcontact fatigue resistance of the rail.

Solution to Problem

We studied to address this issue, and found that optimizing the chemicalcomposition of a rail, and additionally, properly performingstraightening treatment before natural aging is effective to improve the0.2% proof stress of a pearlitic rail. In particular, rails used inheavy haul railways in mines have a long transport period from theshipping to the laying on foreign and domestic mining sites. Thus, basedon the findings that using the period for natural aging is advantageous,we completed the disclosure.

The disclosure is based on the findings described above and has thefollowing primary features.

1. A rail accompanied by a steel material inspection certificate whichdescribes at least a measurement result of a 0.2% proof stress of a headof the rail, having a chemical composition containing (consisting of),in mass%,

C: 0.70% to 0.85%,

Si: 0.1% to 1.5%,

Mn: 0.4% to 1.5%,

P: 0.035% or less,

S: 0.010% or less, and

Cr: 0.05% to 1.50%, with the balance being Fe and inevitable impurities,

wherein the rail exhibits, at least 90 days after a preparation date ofthe steel material inspection certificate, an improvement margin of a0.2% proof stress of 40 MPa or more, relative to the 0.2% proof stressdescribed in the steel material inspection certificate.

2. The rail according to 1, wherein the chemical composition furthercontains, in mass%, at least one selected from the group consisting of

V: 0.30% or less,

Cu: 1.0% or less,

Ni: 1.0% or less,

Nb: 0.05% or less,

Mo: 0.5% or less,

Al: 0.07% or less,

W: 1.0% or less,

B: 0.005% or less, and

Ti: 0.05% or less.

3. A method for producing a rail, comprising:

hot rolling a steel raw material to obtain a rail, the steel rawmaterial having a chemical composition containing (consisting of), inmass %,

C: 0.70% to 0.85%,

Si: 0.1% to 1.5%,

Mn: 0.4% to 1.5%,

P: 0.035% or less,

S: 0.010% or less, and

Cr: 0.05% to 1.50%, with the balance being Fe and inevitable impurities;

straightening the rail with a load of 100 tf or more; and

preparing a steel material inspection certificate including at least ameasurement result of a 0.2% proof stress of a head of the rail within480 hours after the straightening.

4. The method according to 3, wherein the chemical composition furthercontains, in mass%, at least one selected from the group consisting of

V: 0.30% or less,

Cu: 1.0% or less,

Ni: 1.0% or less,

Nb: 0.05% or less,

Mo: 0.5% or less,

Al: 0.07% or less,

W: 1.0% or less,

B: 0.005% or less, and

Ti: 0.05% or less.

Advantageous Effect

According to the disclosure, it is possible to provide a high-strengthpearlitic rail which obtains an excellent 0.2% proof stress by aging andthus can be suitably used in heavy haul railways.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic diagram of a rail head illustrating a collectingposition of a tensile test piece;

FIGS. 2A and 2B are each a schematic diagram of a rail head illustratinga collecting position of a rolling contact fatigue test piece; and

FIG. 3 is a schematic diagram illustrating an overview of bendstraightening of a rail.

DETAILED DESCRIPTION

[Chemical Composition]

Our rail will be specifically explained below. It is important that ourrail has the chemical composition described above. The reasons forlimiting the chemical composition as described above are explainedfirst. The unit of the content of each component is “mass %”, but it isabbreviated as “%”.

C: 0.70% to 0.85%

C is an element that forms cementite in a pearlite structure and has theeffect of improving the 0.2% proof stress by natural aging. Therefore,the addition of C is necessary to ensure the 0.2% proof stress in arail. As the C content increases, the 0.2% proof stress is improved.Specifically, when the C content is less than 0.70%, it is difficult toobtain an excellent 0.2% proof stress after natural aging. On the otherhand, when the C content is beyond 0.85%, pro-eutectoid cementite isformed at prior austenite grain boundaries, ending up deterioratingrolling contact fatigue resistance of a rail. Therefore, the C contentis set to 0.70% to 0.85%, and preferably, 0.75% to 0.85%.

Si: 0.1% to 1.5%

Si is an element that has an effect as a deoxidizer. Further, Si has aneffect of improving the 0.2% proof stress of a rail by solid solutionstrengthening of ferrite in pearlite. Therefore, the Si content needs tobe 0.1 or more. On the other hand, a Si content beyond 1.5% produces alarge amount of oxide-based inclusions because Si has a high strength ofbonding with oxygen, thus deteriorating rolling contact fatigueresistance. Therefore, the Si content is set to 0.1% to 1.5%, andpreferably, 0.15% to 1.5%.

Mn: 0.4% to 1.5%

Mn is an element that improves the strength of a rail by decreasing thetransformation temperature of steel to thereby shorten the lamellarspacing. A Mn content less than 0.4%, however, cannot achieve asufficient effect. On the other hand, a Mn content beyond 1.5% tends togenerate a martensite structure by microsegregation of steel, thusdeteriorating rolling contact fatigue resistance. Therefore, the Mncontent is set to 0.4% to 1.5%, and preferably, 0.4% to 1.4%.

P: 0.035% or less

A P content beyond 0.035% deteriorates ductility of a rail. Therefore,the P content is set to 0.035% or less. On the other hand, the lowerlimit of the P content is not limited, and may be 0%, althoughindustrially more than 0%. Excessively decreasing the P content causesan increase in refining cost. Thus, from the perspective of economicefficiency, the P content is preferably set to 0.001% or more. Morepreferably, the P content is 0.025% or less.

S: 0.010% or less

S exists in steel mainly in the form of an A type (sulfide-based)inclusion. A S content beyond 0.010% significantly increases the amountof the inclusions and generates coarse inclusions, thus deterioratingrolling contact fatigue resistance. Setting the S content to less than0.0005% causes an increase in refining cost. Thus, from the perspectiveof economic efficiency, the S content is preferably set to 0.0005% ormore. More preferably, the S content is 0.009% or less.

Cr: 0.05% to 1.50%

Cr is an element that has an effect of improving the 0.2% proof stressby solid solution strengthening of cementite in pearlite. To achievethis effect, the Cr content needs to be 0.05% or more. On the otherhand, a Cr content beyond 1.50% generates a martensite structure bysolid solution strengthening of Cr, ending up deteriorating rollingcontact fatigue resistance. Therefore, the Cr content is set to 0.05% to1.50%, and preferably 0.10% to 1.50%.

Our rail comprises the aforementioned composition as a steel rawmaterial, with the balance being Fe and inevitable impurities. Thebalance may be Fe and inevitable impurities, and may further contain thefollowing elements within a range which does not substantially affectthe action and effect of the disclosure.

Specifically, the balance may further contain as necessary at least oneselected from the group consisting of

V: 0.30% or less,

Cu: 1.0% or less,

Ni: 1.0% or less,

Nb: 0.05% or less,

Mo: 0.5% or less,

Al: 0.07% or less,

W: 1.0% or less,

B: 0.005% or less, and

Ti: 0.05% or less.

V: 0.30% or less

V is an element that has an effect of precipitating as a carbonitrideduring and after rolling and improving the 0.2% proof stress byprecipitation strengthening. Therefore, 0.001% or more of V ispreferably added. On the other hand, a V content beyond 0.30% causes theprecipitation of a large amount of coarse carbonitrides, thusdeteriorating rolling contact fatigue resistance. Therefore, in the caseof adding V, the V content is preferably set to 0.30% or less.

Cu: 1.0% or less

As with Cr, Cu is an element that has an effect of improving the 0.2%proof stress by solid solution strengthening. Therefore, 0.001% or moreof Cu is preferably added. On the other hand, a Cu content beyond 1.0%causes Cu cracking. Therefore, in the case of adding Cu, the Cu contentis preferably set to 1.0% or less.

Ni: 1.0% or less

Ni has an effect of improving the 0.2% proof stress withoutdeteriorating ductility. Therefore, 0.001% or more of Ni is preferablyadded. In addition, adding Ni along with Cu can prevent Cu cracking.Thus, in the case of adding Cu, Ni is preferably added. On the otherhand, a Ni content beyond 1.0% increases quench hardenability to producemartensite, deteriorating rolling contact fatigue resistance. Therefore,in the case of adding Ni, the Ni content is preferably set to 1.0% orless.

Nb: 0.05% or less

Nb precipitates as a carbonitride during and after rolling and improvesthe 0.2% proof stress of a pearlitic rail. Therefore, 0.001% or more ofNb is preferably added. On the other hand, a Nb content beyond 0.05%causes the precipitation of a large amount of coarse carbonitrides, thusdeteriorating ductility. Therefore, in the case of adding Nb, the Nbcontent is preferably set to 0.05% or less.

Mo: 0.5% or less

Mo precipitates as a carbonitride during and after rolling and improvesthe 0.2% proof stress by precipitation strengthening. Therefore, 0.001%or more of Mo is preferably added. On the other hand, a Mg contentbeyond 0.5% produces martensite, thus deteriorating rolling contactfatigue resistance. Therefore, in the case of adding Mo, the Mo contentis preferably set to 0.5% or less.

Al: 0.07% or less

Al is an element that is added as a deoxidizer. Therefore, 0.001% ormore of Al is preferably added. On the other hand, an Al content beyond0.07% produces a large amount of oxide-based inclusions because Al has ahigh strength of bonding with oxygen, thus deteriorating rolling contactfatigue resistance. Therefore, the Al content is preferably set to 0.07%or less.

W: 1.0% or less

W precipitates as a carbonitride during and after rolling and improvesthe 0.2% proof stress by precipitation strengthening. Therefore, 0.001%or more of W is preferably added. On the other hand, a W content beyond1.0% produces martensite, thus deteriorating rolling contact fatigueresistance. Therefore, in the case of adding W, the W content ispreferably set to 1.0% or less.

B: 0.005% or less

B precipitates as a nitride during and after rolling, and improves the0.2% proof stress by precipitation strengthening. Therefore, 0.0001% ormore of B is preferably added. A B content beyond 0.005% producesmartensite, thus deteriorating rolling contact fatigue resistance.Therefore, in the case of adding B, the B content is preferably set to0.005% or less.

Ti: 0.05% or less

Ti precipitates as a carbide, a nitride, or a carbonitride during andafter rolling, and improves the 0.2% proof stress by precipitationstrengthening. Therefore, 0.001% or more of Ti is preferably added. Onthe other hand, a Ti content beyond 0.05% produces coarse carbides,nitrides, or carbonitrides, thus deteriorating rolling contact fatigueresistance. Therefore, in the case of adding Ti, the Ti content ispreferably 0.05% or less.

[Improvement Margin of a 0.2% Proof Stress of 40 MPa or more]

In the disclosure, it is important that the rail has the aforementionedchemical composition, and additionally exhibits, at least 90 days aftera preparation date of a steel material inspection certificate of therail which describes at least a measurement result of a 0.2% proofstress of a head of the rail, an improvement margin of a 0.2% proofstress of 40 MPa or more, relative to the 0.2% proof stress described inthe steel material inspection certificate.

Specifically, to improve rolling contact fatigue resistance of the rail,the 0.2% proof stress of the rail needs to be improved to limit aplastic deformation area as much as possible. The 0.2% proof stress canbe improved by adding alloying elements, which, however, ratherdeteriorates rolling contact fatigue resistance of the rail by thegeneration of an abnormal structure such as martensite. To prevent thegeneration of an abnormal structure and improve the 0.2% proof stress,straightening treatment and aging treatment under the aforementionedconditions are effective. The 0.2% proof stress at least after thepassage of 90 days can be improved by straightening treatment underoptimal loads and natural aging for an optimal period

When the improvement margin of a 0.2% proof stress is as small as lessthan 40 MPa, a plastic flow is easily caused on a surface of the rail,and fatigue layers easily accumulate on a surface of the rail,decreasing the improvement margin of rolling contact fatigue resistance.Therefore, the improvement margin of a 0.2% proof stress is set to 40MPa or more. As used herein, the “improvement margin of a 0.2% proofstress” can be determined as a difference between a 0.2% proof stress ofthe rail at a preparation date of a “steel material inspectioncertificate” after production of the rail, specifically, almost at thetime of shipping (hereinafter, referred to as before-aging rail) and a0.2% proof stress obtained by performing a tensile test on test piecescollected from the rail at least 90 days after the preparation date(hereinafter, referred to as aged rail) (a 0.2% proof stress of the agedrail - a 0.2% proof stress of the before-aging rail).

The improvement margin of a 0.2% proof stress at least after the passageof 90 days is used to evaluate a 0.2% proof stress of the rail which hassufficiently exhibited strain aging, in which C is stuck to strainsintroduced into the rail in rail straightening, thus improving the 0.2%proof stress.

The steel material inspection certificate describes a result of testingmechanical properties on the rail obtained through a final process ofproducing the rail. The rail is shipped to a customer attached with thesteel material inspection certificate. The rail of the disclosure isattached with the steel material inspection certificate, which describesat least a measurement result of a 0.2% proof stress of a head of therail. The described measurement result of a 0.2% proof stress is a valueobtained in a tensile test on samples collected from a head of the rail.Further, at least 90 days after a preparation date of the steel materialinspection certificate, the 0.2% proof stress is improved by 40 MPa ormore, relative to the 0.2% proof stress described in the steel materialinspection certificate. Thus, considering that there are typically 90days or more from the shipping of the rail to the rail laying, the railobtains more improved rolling contact fatigue resistance than expectedfrom the 0.2% proof stress described in the steel material inspectioncertificate. Even if the time from a preparation date of the steelmaterial inspection certificate to the rail laying is less than 90 days,the rail typically has a service life substantially longer than 90 days.Thus, during use of the rail, 90 days pass from the preparation date ofthe steel material inspection certificate before rolling contact fatigueoccurs in the rail, and thus the 0.2% proof stress is increased,improving rolling contact fatigue resistance.

The improvement margin of a 0.2% proof stress may be measured at least90 days after a preparation date of the steel material inspectioncertificate and does not need to be measured in 90 days from apreparation date. In other words, the improvement margin of a 0.2% proofstress may be measured in 90 days from a preparation date, or 1 year ormore after a preparation date of the steel material inspectioncertificate.

The improvement of a 0.2% proof stress of 40 MPa or more may beconsidered as a safety margin for a predicted value of the rail servicelife. Alternatively, 40 MPa is further added to a 0.2% proof stressdescribed in the steel material inspection certificate and the obtainedvalue may be used to predict the rail service life.

[Producing Conditions]

Next, a method for producing our rail will be described.

Our rail can be produced by making a rail through hot rolling andcooling according to a usual method and subsequently subjecting the railto straightening treatment with loads of 100 tf or more.

The rail is produced by hot rolling, for example, in accordance with thefollowing procedures.

First, steel is melted in a converter or an electric heating furnace andsubjected as necessary to secondary refining such as degassing.Subsequently, the chemical composition of the steel is adjusted withinthe aforementioned range. Next, the steel is subjected to continuouscasting to make a steel raw material such as bloom. Subsequently, thesteel raw material is heated in a heating furnace to 1200° C. to 1350°C. and hot rolled to obtain a rail. The hot rolling is preferablyperformed at rolling finish temperature: 850° C. to 1000° C. and therail after the hot rolling is preferably cooled at cooling rate: 1° C./sto 10° C./s.

After the cooling following the hot rolling is finished, the rail issubjected to straightening treatment with loads of 100 tf or more tostraighten a bend of the rail. The bend of the rail is straightened bypassing the rail through straightening rollers disposed in zigzag alongthe feed direction of the rail and subjecting the rail to repeatedbending/bend restoration deformation. FIG. 3 is a conceptual diagramillustrating a method for straightening a bend of the rail. The bendstraightening of a rail is performed by passing a rail R throughstraightening rollers A to G disposed in zigzag along the feed directionof the rail. In FIG. 3, top surfaces of straightening rollers A, B, andC disposed below the feed line are arranged at an upper side than bottomsurfaces of straightening rollers D, E, F, and G disposed above the feedline. By passing the rail through the straightening roller group, therail is subjected to bending/bend restoration deformation. During thestraightening, at least one of straightening loads applied to thestraightening rollers A to G is 100 tf or more. For example, in theexample of FIG. 3, seven straightening rollers in total, that is, threestraightening rollers in the lower side of the figure and fourstraightening rollers in the upper side of the figure are applied withstraightening loads of F_(A), F_(B), F_(C), F_(D), F_(E), F_(F), andF_(G), among which, the largest straightening load is 100 tf or more. Bysetting the straightening load to 100 tf or more, strains can beintroduced into the rail so that the improvement margin of a 0.2% proofstress may be 40 MPa after the passage of 90 days. The straighteningload is preferably 105 tf or more and 170 tf or less.

Strains accumulated in the rail by straightening treatment is changeddepending on the straightening load and the cross-sectional area of therail (size of the rail) to be subjected to the straightening treatment.Here, the rail to be used under high axle load conditions which ismainly targeted in the disclosure has a size of 115 lbs, 136 lbs, and141 lbs in the North America AREMA Standard which has a relatively largecross-section, and a size of 50 kgN and 60 kgN in the JIS Standard. Whenthe rail having such a size is applied with a straightening load of 100tf or more, enough strains can be accumulated in the rail to produce asufficient improvement margin of a 0.2% proof stress relative a 0.2%proof stress measured within 480 hours after the straightening.

Within 480 hours after the bend straightening of the rail is performed,a steel material inspection certificate is prepared which includes atleast the measurement result of a 0.2% proof stress of a head of therail. When material properties of the rail including a 0.2% proof stressare inspected after a long period of time which causes strain aging haspassed after the bend straightening of the rail, the 0.2% proof stressis found to have been increased relative to that inspected immediatelyafter the straightening. Thus, the rail can be shipped as a rail havinga high 0.2% proof stress. However, storing the rail in factories for along period of time after the straightening treatment is impossiblebecause of limitation of the extent of a repository. Therefore, thesteel material inspection certificate which describes at least themeasurement result of a 0.2% proof stress of a head of the rail isprepared immediately after the bend straightening, that is, within 480hours after the bend straightening.

A rail made from a steel raw material having the aforementioned chemicalcomposition exhibits an improvement margin of a 0.2% proof stress of 40MPa or more at least 90 days after a preparation date of the steelmaterial inspection certificate by virtue of natural aging for at least90 days after the straightening treatment.

EXMAMPLES Example 1

Steel raw materials (bloom) having a chemical composition listed inTable 1 were hot rolled to obtain rails having a size listed in Table 2.At that time, the heating temperature before the hot rolling was 1250°C., and the delivery temperature was 900° C. The hot-rolled rails werecooled to 400° C. at an average cooling rate of 3° C./s. Subsequently,the cooled rails were subjected to straightening treatment underconditions listed in Table 2.

A tensile test was performed on the obtained rails to measure their 0.2%proof stress, tensile strength, and elongation. Further, a rollingcontact fatigue resistance test was performed to measure rolling contactfatigue resistance of the rails. Table 2 also lists these results. Themeasurement method was as follows. The tensile test was performedbetween the straightening treatment and the preparation of a steelmaterial inspection certificate. Additionally, the tensile test was alsoperformed in the rails other than No. 1 after natural aging treatment.

[Tensile Test]

For heads of the obtained rails, tensile test pieces were collected fromthe portion illustrated in FIG. 1. Specifically, tensile test pieceshaving a diameter of parallel portion as described in ASTM A370 of 12.7mm were collected from a position described in 2.1.3.4 of Chapter 4 ofAREMA (see FIG. 1). Next, using the obtained tensile test pieces, atensile test was performed under conditions of a tension speed of 1mm/min and a gauge length of 50 mm to measure 0.2% proof stress, tensilestrength, and elongation. The measurement results were listed in Table2.

The tensile test was performed within 480 hours after the straighteningtreatment on test pieces of heads of the rails collected fromimmediately after (within 480 hours after) the straightening treatment.As to the rails other than No. 1, the tensile test was also performed 90days after the preparation of a steel material inspection certificate.Further, test pieces were collected from heads of the rails after therails had gone through natural aging treatment for a natural agingtreatment period as listed in Table 2. On the test pieces, a tensiletest was performed after time (days) from the preparation of a steelmaterial inspection certificate to the tensile test as listed in Table 2had passed. Then, the improvement margin (MPa) of a 0.2% proof stressafter natural aging treatment was measured, relative to the 0.2% proofstress measured in the tensile test immediately after the straighteningtreatment.

[Rolling Contact Fatigue Resistance]

Rolling contact fatigue resistance was evaluated using a Nishihara typewear test apparatus and simulating actual contact conditions between arail and a wheel. Specifically, cylinder test pieces having a diameterof 30 mm (an outer diameter of 30 mm and an inner diameter of 16 mm)with a contact surface being a curved surface having a radius ofcurvature of 15 mm were collected from heads of the rails after naturalaging treatment as illustrated in FIG. 2A. The cylinder test pieces werefed to the test apparatus as illustrated in FIG. 2B with a contactpressure of 2.2 GPa and a slip rate of −20% under oil lubricationconditions. At the time when spalling occurred in a contact surface ofthe test pieces, the test pieces were determined to have reached theirrolling contact fatigue life. As a standard when comparing the rollingcontact fatigue life, an actually-used pearlite steel rail having the Ccontent of 0.81% was adopted. When the rolling contact fatigue time was10% or more longer than in the actually-used pearlite steel rail (A1),the rolling contact fatigue resistance was determined to have beenimproved.

The wheel material illustrated in FIGS. 2A and 2B was subjected to thetest, the wheel material being obtained by heating a round bar with adiameter of 33 mm to 900° C., the bar having a chemical compositioncontaining, in mass%, 0.76% C, 0.35% Si, 0.85% Mn, 0.017% P, 0.008% S,and 0.25% Cr with the balance being Fe and inevitable impurities,holding the bar for 40 minutes, subsequently allowing it to be naturallycooled, and forming it into a wheel material as illustrated in FIG. 2B.The hardness of the wheel material was HV280.

TABLE 1 Steel sample Chemical composition (mass %)* ID C Si Mn P S CrRemarks A1 0.81 0.25 1.18 0.009 0.005 0.25 Conforming Steel A2 0.84 0.510.62 0.011 0.004 0.77 Conforming Steel A3 0.69 0.24 0.82 0.008 0.0070.15 Comparative Steel *The balance is Fe and inevitable impurities

TABLE 2 Measurement results Time from straightening After straighteningtreatment to (described in steel material After aging treatment 1*¹preparation of steel inspection certificate) 0.2% Improvement SteelStraightening material inspection 0.2% proof proof margin of 0.2% sampleload certificate stress Tensile strength Elongation stress proof stressNo. ID Size (tf) (h) (Mpa) (MPa) (%) (Mpa) (MPa) 1 A1 136 lbs 100 24 9201404 12.0 — — 2 A2 141 lbs  95 24 932 1432 12.1 935  3 3 A2 141 lbs 15024 934 1432 12.5 985 51 4 A2  50 kgN 150 30 931 1433 12.3 973 42 5 A2136 lbs 150 30 931 1440 12.5 977 46 6 A2 141 lbs 140 30 933 1439 12.6974 41 7 A2 136 lbs 140 24 934 1432 12.5 985 51 8 A2 136 lbs 120 48 9311433 12.7 980 49 9 A2  50 kgN 120 48 931 1433 12.8 978 47 10 A2 136 lbs180 40 932 1433 12.5 982 50 11 A3 141 lbs 140 40 892 1387 12.7 910 18 12A3 136 lbs 140 40 888 1389 12.8 922 34 13 A2 136 lbs 120 480 935 142812.5 981 46 14 A2 136 lbs 120 240 930 1428 12.8 975 45 15 A2 141 lbs 100480 925 1430 12.5 974 49 16 A2  50 kgN  95 48 925 1430 12.8 926  1Measurement results After aging treatment 2*² Improvement Treatment Timefrom preparation margin of period of date of steel material 0.2%Improvement rolling contact natural inspection certificate to proofTensile margin of 0.2% fatigue aging tensile test stress strengthElongation proof stress resistance No. (days) (days) (Mpa) (MPa) (%)(MPa) (%) Remarks 1 — — — — — — Standard Comparative Example 2 90.8 92.0935 1445 12.2  3  2 Comparative Example 3 118.8 120.0 981 1451 12.5 4714 Example 4 90.5 92.0 972 1421 14.7 41 16 Example 5 98.5 100.0 979 130715.2 48 19 Example 6 93.5 95.0 974 1288 15.6 41 17 Example 7 148.8 150.0993 1439 12.6 59 26 Example 8 117.6 120.0 988 1434 12.7 57 22 Example 9137.6 140.0 988 1422 12.6 57 24 Example 10 118.0 120.0 984 1435 12.5 5218 Example 11 118.0 120.0 911 1453 12.1 19  5 Comparative Example 12118.0 120.0 922 1399 12.8 34  9 Comparative Example 13 118.0 120.0 9831441 12.5 48 15 Example 14 118.0 120.0 980 1430 13.0 50 16 Example 15118.0 120.0 975 1422 13.1 50 13 Example 16 90.8 92.0 925 1431 12.8  0  2Comparative Example *¹After the preparation of a steel materialinspection certificate, natural aging is done for 90 days, and then atensile test is performed. *²After the preparation of a steel matetrialinspection certificate, natural aging is done for a treatment period ofnatural aging listed in Table 2, and then a tensile test is performed.

The rail of Comparative Example No. 1 in Example 1 was an actually-usedpearlitic rail having the C content of 0.81%. As seen from the resultslisted in Table 2, rails of Examples according to the disclosure had amore excellent 0.2% proof stress than the rail of Comparative ExampleNo. 1 by 40 MPa or more and exhibited an improvement margin of rollingcontact fatigue resistance of 10% or more. On the other hand, the railsof Comparative Examples which did not satisfy the conditions of thedisclosure were inferior in at least one of a 0.2% proof stress,elongation, and rolling contact fatigue resistance.

Example 2

Rails were made in the same procedures as in Example 1 other than usingsteel having a chemical composition listed in Table 3. A tensile testand measurement of rolling contact fatigue resistance were performed onthe rails in the same way as in Example 1. Table 4 lists conditions ofstraightening treatment and aging treatment, and measurement results.

As seen from the results listed in Table 4, the rails of Examplessatisfying the conditions of the disclosure had a more excellent 0.2%proof stress than the rail of Comparative Example No. 1 by 40 MPa ormore and exhibited an improvement margin of rolling contact fatigueresistance of 10% or more. On the other hand, the rails of ComparativeExamples which did not satisfy the conditions of the disclosure wereinferior in at least one of a 0.2% proof stress and rolling contactfatigue resistance.

TABLE 3 Steel sample Chemical Composition (mass %)* ID C Si Mn P S Cr CuNi Mo V Nb Al W B Ti Remarks A1 0.81 0.25 1.17 0.011 0.006 0.25 — — — —— — — — — Conforming Steel B1 0.83 1.50 0.48 0.014 0.007 0.26 — — — — —— — — — Conforming Steel B2 0.83 0.25 0.85 0.005 0.007 0.61 — — — — — —— — — Conforming Steel B3 0.70 0.42 0.40 0.003 0.006 1.50 — — — — — — —— — Conforming Steel B4 0.84 0.88 0.46 0.016 0.005 0.79 — — — — — — — —— Conforming Steel B5 0.83 0.87 0.47 0.003 0.006 1.46 — — — — — — — — —Conforming Steel B6 0.84 0.22 1.20 0.005 0.007 0.21 — — — — — — — — —Conforming Steel B7 0.81 0.69 0.56 0.015 0.007 0.79 — — — — — — — — —Conforming Steel B8 0.71 1.16 1.34 0.016 0.004 0.88 — — — — — — — — —Conforming Steel B9 0.84 1.06 0.83 0.019 0.006 0.05 — — — — — — — — —Conforming Steel B10 0.85 0.48 0.71 0.016 0.004 0.32 — — — — — — — — —Conforming Steel B11 0.68 0.25 0.81 0.015 0.006 0.05 — — — — — — — — —Comparative Steel B12 0.86 0.24 0.81 0.015 0.007 0.22 — — — — — — — — —Comparative Steel B13 0.72 0.04 0.81 0.015 0.005 0.21 — — — — — — — — —Comparative Steel B14 0.82 1.55 0.82 0.014 0.005 0.99 — — — — — — — — —Comparative Steel B15 0.72 0.25 0.34 0.015 0.005 0.18 — — — — — — — — —Comparative Steel B16 0.84 0.29 1.55 0.011 0.005 0.99 — — — — — — — — —Comparative Steel B17 0.81 0.63 0.81 0.006 0.003 0.01 — — — — — — — — —Comparative Steel B18 0.85 0.59 0.81 0.007 0.003 1.55 — — — — — — — — —Comparative Steel B19 0.84 0.55 0.55 0.014 0.005 0.79 — — — 0.05 — — — —— Conforming Steel B20 0.84 0.51 0.61 0.008 0.004 0.74 — — — 0.15 — — —— — Conforming Steel B21 0.84 0.25 1.10 0.006 0.005 0.25 — — — — 0.04 —— — — Conforming Steel B22 0.84 0.35 1.05 0.003 0.004 0.29 — — 0.30 — —— — — — Conforming Steel B23 0.84 0.55 0.55 0.011 0.005 0.62 0.30 0.50 —— — — — — — Conforming Steel B24 0.84 0.25 1.20 0.004 0.005 0.29 — — — —— 0.07 0.60 — — Conforming Steel B25 0.84 0.88 0.55 0.005 0.005 0.45 — —— — — — — 0.003 0.05 Conforming Steel B26 0.84 0.95 0.56 0.011 0.0050.79 — — — 0.05 — — — — — Conforming Steel *The balance is Fe andinevitable impurities

TABLE 4 Measurement results Time from straightening After straighteningtreatment to (described in steel material After aging treatment 1*¹preparation of steel inspection certificate) Improvement SteelStraightening material inspection 0.2% proof Tensile 0.2% proof marginof 0.2% sample load certificate stress strength Elongation stress proofstress No. ID Size (tf) (h) (Mpa) (MPa) (%) (Mpa) (MPa) 17 A1  50 kgN —24 920 1404 12.0 — — 18 B1 136 lbs 120 24 933 1442 12.2 975 42 19 B2 141lbs 140 24 929 1431 12.2 969 40 20 B3 141 lbs 150 30 887 1387 13.1 92841 21 B4 136 lbs 150 35 933 1433 12.8 981 48 22 B5 141 lbs 140 48 9521441 12.3 993 41 23 B6 136 lbs 140 24 918 1398 11.7 959 41 24 B7 136 lbs130 35 929 1422 12.5 972 43 25 B8 141 lbs 135 35 929 1423 12.6 970 41 26B9 136 lbs 140 35 934 1439 12.6 976 42 27 B10 141 lbs 150 30 929 142212.3 978 49 28 B11 136 lbs 130 24 889 1377 12.4 921 32 29 B12 136 lbs130 24 948 1421 11.1 988 40 30 B13 136 lbs 130 24 892 1387 12.2 931 3931 B14 141 lbs 130 24 944 1429 12.3 984 40 32 B15 136 lbs 130 24 8891387 12.3 919 30 33 B16 141 lbs 130 24 921 1428 12.4 961 40 34 B17 136lbs 130 24 879 1399 12.2 915 36 35 B18 136 lbs 130 24 922 1432 12.3 96442 36 B19 141 lbs 200 24 933 1433 12.4 980 47 37 B20 141 lbs 150 30 9421439 12.5 986 44 38 B21 136 lbs 140 48 934 1433 12.1 982 48 39 B22 136lbs 140 24 929 1438 12.0 971 42 40 B23 136 lbs 130 28 941 1432 12.3 98443 41 B24 141 lbs 120 28 923 1430 12.2 967 44 42 B25  50 kgN 100 29 9231439 12.2 966 43 43 B26 136 lbs 140 28 931 1423 12.3 975 44 44 B25 141lbs 100 120 940 1424 12.5 980 40 45 B26 136 lbs 140 240 938 1430 13.0983 45 46 B26 141 lbs 100 480 935 1430 12.8 985 50 47 B26  50 kgN 100480 943 1451 12.2 998 55 Measurement results After aging treatment 2*²Improvement Treatment Time from a preparation margin of period of dateof steel material 0.2% Improvement rolling contact natural inspectioncertificate to proof Tensile margin of 0.2% fatigue aging tensile teststress strength Elongation proof stress resistance No. (days) (days)(Mpa) (MPa) (%) (MPa) (%) Remarks 17 — — — — — — Standard ComparativeExample 18 118.8 120.0 974 1435 12.4 41 11 Example 19 90.8 92.0 969 143812.3 40 13 Example 20 93.5 95.0 928 1389 12.9 41 11 Example 21 118.3120.0 982 1432 12.7 49 14 Example 22 117.6 120.0 995 1442 12.3 43 13Example 23 148.8 150.0 960 1423 11.1 42 13 Example 24 148.3 150.0 9741429 12.2 45 14 Example 25 118.3 120.0 971 1423 12.4 42 15 Example 26128.3 130.0 976 1438 12.5 42 12 Example 27 133.5 135.0 980 1430 12.4 5116 Example 28 98.8 100.0 921 1387 12.3 32  9 Comparative Example 29 98.8100.0 989 1420 10.7 41  9 Comparative Example 30 98.8 100.0 931 138912.2 39  9 Comparative Example 31 98.8 100.0 984 1430 12.3 40  9Comparative Example 32 118.8 120.0 920 1392 12.5 31  7 ComparativeExample 33 108.8 110.0 963 1429 12.4 42  8 Comparative Example 34 108.8110.0 917 1401 12.2 38  8 Comparative Example 35 98.8 100.0 965 143312.3 43  7 Comparative Example 36 148.8 150.0 984 1430 12.4 51 15Example 37 148.5 150.0 988 1433 12.2 46 11 Example 38 137.6 140.0 9831435 12.1 49 13 Example 39 138.8 140.0 972 1439 12.4 43 11 Example 40138.6 140.0 985 1433 12.3 44 12 Example 41 128.6 130.0 968 1439 12.4 4514 Example 42 148.6 150.0 968 1440 12.5 45 14 Example 43 148.6 150.0 9791433 12.3 48 12 Example 44 145.0 150.0 981 1435 12.5 41 12 Example 45138.8 150.0 985 1440 12.2 47 13 Example 46 140 150.0 989 1442 12.5 54 13Example 47 140 150.0 909 1472 12.2 56 15 Example *¹After the preparationof a steel material inspection certificate, natural aging is done for 90days, and then a tensile test is performed. *²After the preparation of asteel matetrial inspection certificate, natural aging is done for atreatment period of natural aging listed in Table 2, and then a tensiletest is performed.

1. A rail accompanied by a steel material inspection certificate which describes at least a measurement result of a 0.2% proof stress of a head of the rail, having a chemical composition containing, in mass%, C: 0.70% to 0.85%, Si: 0.1% to 1.5%, Mn: 0.4% to 1.5%, P: 0.035% or less, S: 0.010% or less, and Cr: 0.05% to 1.50%, with the balance being Fe and inevitable impurities, wherein the rail exhibits, at least 90 days after a preparation date of the steel material inspection certificate, an improvement margin of a 0.2% proof stress of 40 MPa or more, relative to the 0.2% proof stress described in the steel material inspection certificate.
 2. The rail according to claim 1, wherein the chemical composition further contains, in mass%, at least one selected from the group consisting of V: 0.30% or less, Cu: 1.0% or less, Ni: 1.0% or less, Nb: 0.05% or less, Mo: 0.5% or less, Al: 0.07% or less, W: 1.0% or less, B: 0.005% or less, and Ti: 0.05% or less.
 3. A method for producing a rail, comprising: hot rolling a steel raw material to obtain a rail, the steel raw material having a chemical composition containing, in mass %, C: 0.70% to 0.85%, Si: 0.1% to 1.5%, Mn: 0.4% to 1.5%, P: 0.035% or less, S: 0.010% or less, and Cr: 0.05% to 1.50%, with the balance being Fe and inevitable impurities; straightening the rail with a load of 100 tf or more; and preparing a steel material inspection certificate including at least a measurement result of a 0.2% proof stress of a head of the rail within 480 hours after the straightening.
 4. The method according to claim 3, wherein the chemical composition further contains, in mass%, at least one selected from the group consisting of V: 0.30% or less, Cu: 1.0% or less, Ni: 1.0% or less, Nb: 0.05% or less, Mo: 0.5% or less, Al: 0.07% or less, W: 1.0% or less, B: 0.005% or less, and Ti: 0.05% or less. 